Dear Paul, LF Group,
For Stefan's benefit, 1' = 0.3048m, so we are talking about a 52m long wire.
I measured my home QTH inv-L antenna (about 55m of wire in total, about 10m
height) at 10kHz to have a capacitance of about 340pF, in series with a
resistance of about 300ohms. This is in a location with several small trees,
etc., so you could possibly expect lower resistance in an open location, or
higher if you are in a forest!
As has been pointed out, it depends on the type of bridge whether you are
measuring a series or parallel RC equivalent - for the above antenna, the
parallel equivalent would be about 340pF/7.3Megohms. The magnitude of the
impedance at 9kHz would be about 52kohms
These are much higher impedances than are commonly measured using amateur RF
bridges. If you look at old textbooks on AC bridge measurements, you will
see elaborate shielding/guarding measures are often required for high
impedance measurements. The basic problem is you are trying to measure a
small resistance component while balancing out a much larger, quadrature,
capacitive component. So anything that affects the balance of the bridge
(e.g. stray coupling between the components, or source or detector), can
cause large errors. I imagine good electrostatic shielding between the
bridge components and antenna wire would be essential. A first step would be
to check the bridge works accurately on some known impedances (e.g. a
low-loss capacitor of a few 100pF with a series resistor).
The way I did my measurements was to resonate the antenna at the measurement
frequency with a series inductor, then measure the relatively low resistive
impedance of the combination with a simple resistance bridge. The loss
resistance of the coil was measured by replacing the antenna with a
calibrated air-variable capacitor, measuring the resistance again, and
subtracting it from the antenna+coil resistance. This still has some
potential errors (e.g. due to the stray capacitance of the inductor to
ground) but avoids the need to resolve the R/C components, and reflects the
way the antenna will actually be used.
Cheers, Jim Moritz
73 de M0BMU
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